1. Field of the Invention
The present invention relates to a control system for the operation of a transformer, such as would be found in, for example, an electrophotographic printer.
In machines wherein a wide range of voltages are required simultaneously, the safe and efficient use of transformers represents a significant design problem. In an electrophotographic printer, the various electronic elements may operate at ranges including 5 volts for digital control circuits, 110 volts for motors, and as much as 1000 volts for charging devices. Various transformers are typically required in making such voltages available from an original power source. With a step-up or step-down transformer, however, there are well-known problems of efficiency and safety. Whenever induction coils are used, enormous incidental voltages can result within the system because of the relationship V=L di/dt; the rapid switching of, for example, a digitally-controlled inverter may create very small values of dr, which results in a very high value of di/dt, in turn resulting in uncontrollably high voltage spikes. These voltage spikes can, of course, create arcing and consequential damage to the circuit and the machine. There is therefore a need for control systems which facilitate manageable control of the power associated with transformers.
2. Description of the Prior Art
U.S. Pat. No. 3,894,280 discloses a transformer in combination with a push-pull transistor switch which forms the inverter of a DC to DC converter. A controllable current source responsive to the converter output voltage controls the charging of a timing capacitor which upon reaching a threshold value causes termination of each inverter half-cycle.
U.S. Pat. No. 4,710,859 discloses a DC converter in which a switching transistor forming the inverter for a transformer is controlled by tuning an oscillating circuit. By tuning this oscillating circuit, the switching transistor off time and duty cycle is varied within large ranges.
U.S. Pat. No. 4,785,387 discloses a transformer having a characteristic leakage inductance. A capacitor of a characteristic capacitance is coupled to the secondary coil to form a resonant circuit including the leakage inductant of the transformer and the capacitor. A rectifying circuit couples the capacitor to a load whereby the voltage stored in the capacitor is delivered to the load.
U.S. Pat. No. 4,868,729 discloses a power supply unit for an electrostatic printing apparatus. A transformer is driven by a drive circuit having a pulse generator and produces a high voltage in the course of decay of a pulse. An output of a secondary winding of the transformer is detected to control the drive circuit such that the output of the secondary winding is kept at a constant level. A discrete capacitor is placed in series with the primary coil of the transformer to form a series-resonant tank circuit; as seen in FIG. 4 of the patent, the effect is to create a sinusoidal voltage behavior and a ramp current behavior with each switching pulse.
U.S. Pat. No. 4,920,471 discloses a high voltage AC power supply which includes a sinusoidal transformer having a high turn winding and a sampling circuit on its secondary side for sampling voltage proportional to the load current. The sampled voltage is compared to a reference voltage and the error voltage is supplied to a buck regulator. The buck regulator converts the error voltage to a variable DC voltage which is fed to an oscillator formed by the primary winding of the transformer.
U.S. Pat. No. 5,003,230 discloses a fluorescent lamp controller with an isolation transformer. The controller applies the high frequency current to the primary winding of the transformer. The secondary winding is coupled to a control input via loading circuitry. A detector circuit supplies the signal from the lamp controller indicative of the results of loading of the transformer to control lamp intensity.
U.S. Pat. No. 5,111,118 discloses a controller for a half-bridge DC/AC converter in a frequency range which is offset from a resonant frequency of an output circuit which includes a transformer and capacitor and which couples the converter to a fluorescent lamp load.
U.S. Pat. No. 5,274,704 discloses a transformer having first, second and third windings. The input to which a signal receiving line is connected is an inverting input, and a first resistor is connected in series between the receive line and the inverting input. The third winding is connected in series with a second resistor to form a feedback loop which is connected to the inverting input of the amplifier. The transfer function of the interface circuit which includes the transformer is dependent on the ratio of the resistances of the first and second resistors and the turns ratio of the first winding to the third winding but is intended to be substantially independent of transformer parasitics.